maintainers: update seylerius's github

maintainers/maintainer-list.nix

···
   seylerius = {
     name = "Sable Seyler";
     email = "[e-mail]";
-    github = "seylerius";
+    github = "sableseyler";
     githubId = 1145981;
     keys = [ { fingerprint = "7246 B6E1 ABB9 9A48 4395  FD11 DC26 B921 A9E9 DBDE"; } ];
   };